It is well known that an increase in resolution of optical projection lithography is achieved primarily by shortening exposure wavelength and increasing numerical aperture. However, as the exposure wavelength is shortened and the numerical aperture is increased, effective depth of focus (DOF) for a projection object lens will decrease dramatically. Although some resolution-enhancing techniques, such as off-axis lamination and phase-shift mask, have been used to alleviate the decrease of DOF, the trend towards a decreased DOF still dominate with the increase of the resolution. The DOF is typically around only 300 nm in the mainstream 193 nm lithography. The fact that the actual DOF does not reach the DOF tolerance required by the lithography process will adversely affect exposure line quality and IC yield. Accordingly, it is very important to make full use of the effective DOF for efficient exposure of a wafer, and to this end, various focus detection apparatuses have been proposed. The focus detection apparatus in a projection lithography machine is configured to measure height and inclination of a certain surface region of the wafer. Then, the wafer is subjected to leveling and focusing such that the exposure region of the wafer surface is positioned within the effective DOF range of the projection object lens, thereby achieving an efficient exposure of the wafer.
The existing focus detection apparatuses are divided, in terms of operation principle, into three categories including capacitive, optical and pneumatic focus detection. The capacitive focus detection technique was mainly used for stepping-type projection lithography machines in earlier days. Now such technique is no longer in use due to some problems with accuracy, process applicability and the like. The optical focus detection technique dominates in the current stepping scan lithography machines provided by major manufactures. The optical focus detection technique includes luminosity, CCD, laser interference and grating focus detections. The luminosity focus detection technique is simple in principle and easy in implementation, but has a low precision in the range of several hundred nanometers and significant non-linearity. The CCD focus detection technique can achieve a repetitive test precision up to about 30 nm by using an array of slits or apertures in cooperation with broadband illumination, and is widely used. The laser interference focus detection technique can achieve a repetitive test precision of 20 nm. However, this technique requires complex graphic processing algorithms and is not good in real-time application, and thus has a limited use. The grating focus detection technique enables measurement over a large area of wafer surface by using various gratings as focus detection marks. This can effectively smooth the effects caused by undulation of the wafer topography and variation in emissivity. Meanwhile, utilization of polarization modulation and Moire technique extends dynamic measurement range of a sensor and reduces influence of light intensity fluctuation. The grating focus detection technique can achieve a high precision. However, the precision suffers from multi-beam interference caused by a layer of photoresist mask, wavefront distortion caused by substrate dielectric medium and the like. The pneumatic focus detection is operated in an aerodynamic ranging principle, and can achieve a sensitivity of sub-nanometer order. This technique, however, has some disadvantages, such as inapplicability in a vacuum environment, direct reading conversion in which no air gap can pass through the airflow sensor, and slow-speed scanning, which limits application of the technique.